a) Field of the Invention
The present invention relates to a field emission element, and more particularly to manufacturing technologies for a field emission element having a field emission cathode from the tip of which electrons are emitted.
b) Description of the Related Art
A field emission element emits electrons from a sharp tip of an emitter (electron emission cathode) by utilizing electric field concentration. For example, a flat panel display can be structured by using a field emitter array (FEA) having a number of emitters disposed on the array. Each emitter controls the luminance of a corresponding pixel of the display.
In order to apply an electric field to the emitter tip of a field emission element and emit electrons therefrom, the gate electrode biased to a positive potential relative to the emitter (cathode) is disposed near the emitter electrode.
An increase in an emission current from the emitter (i.e., a lowered threshold voltage between the gate and emitter), a high speed drive, and the like are requisites for a field emission element. In order to provide these requisites, it is necessary to devise the structure and shape of a field emission element. It is also necessary to devise a manufacture method in order to manufacture such a field emission element stably and reliably.
A low threshold voltage between the gate and emitter can be achieved by making the distance between the emitter and gate electrode surfaces as gate hole of a gate electrode 100 has a rectangular cross section. There is a danger of a short circuit between the gate and emitter electrodes if a distance between the surface of the emitter electrode 110 and the upper edge of the gate electrode 100 is made short. From this reason, the gate electrode 100 cannot be disposed too near the emitter electrode 110.
If a thickness t of the gate electrode 100 is made large, an electric field at the tip of the emitter electrode 110 becomes high so that the threshold voltage between the gate and emitter can be lowered without reducing an emission current. In other words, a larger emission current can be obtained without raising a voltage between the gate and emitter. Further, if the thickness t of the gate electrode 100 is made large, the wiring resistance of the gate electrode 100 becomes low so that a high speed drive becomes possible. However, if the thickness t of the gate electrode 100 having a rectangular cross section of the gate hole such as shown in FIG. 16 is made large, the danger of the short circuit at the upper edge of the gate electrode 100 increases. In order to avoid this, it is necessary to set a longer distance between the tip surface of the emitter electrode 110 and the surface of the gate electrode 100 by reducing the thickness t. It is therefore difficult to lower the threshold voltage.
Another requisite for a field emission element is a sharp tip of the emitter electrode 110. Namely, if an apex angle of the tip of the emitter electrode 110 is made small, the electric field at the tip of the emitter electrode 110 becomes high. It is therefore possible to lower a threshold voltage between the gate and emitter without reducing an emission current, and hence obtain a large emission current without raising a voltage between the gate and emitter. From this reason, it is an important issue to adopt a manufacture method capable of easily sharpening the tip of an emitter electrode 110 having a desired shape.